The invention relates to an organic electroluminescent component, in particular a light-emitting diode (LED) for luminous signs, luminaires, solid-state image intensifiers, or picture screens, with a layer arrangement comprising a first electrode layer, an inorganic layer which conducts electrons, one or several optoelectronically active layers with at least one light-emitting layer which comprises an organic emitter, and a second electrode layer.
Prior-art LEDs are usually inorganic semiconductor diodes, i.e. diodes whose emitter material is an inorganic semiconductor such as doped zinc sulphide, silicon, germanium, or III-V semiconductors, for example InP, GaAs, GaAlAs, GaP, or GaN with suitable dopants.
Work has been going on for several years in the development of luminescent radiation sources in which the emitter material is not an inorganic semiconductor but an organic electrically conductive material.
Electroluminescent components with light-emitting layers built up from organic materials are clearly superior to light sources made from inorganic materials in a number of respects. An advantage is their easy plasticity and high elasticity which opens new possibilities for applications such as luminous signs and picture screens. These layers may readily be manufactured as large-area, flat, and very thin layers which in addition require little material. They excel through their remarkably high brightness accompanied by low operating voltages.
In addition, the color of the emitted light can be varied over a wide range from approximately 400 nm up to approximately 650 nm through the choice of the luminescent material. These colors have a striking luminance.
Such organic electroluminescent components may be built up in various ways. They all have in common that one or several optoelectronically active organic layers, among which the light-emitting layer, are arranged between two electrode layers to which the voltage necessary for operating the component is applied. At least one of the electrode layers is transparent to visible light so that the emitted light can emerge to the exterior. The entire layer construction is usually provided on a substrate which is also transparent to visible light if the emitted light is to issue from the side facing the substrate.
The layer sequence of the optoelectronically active organic layers is known in several variations. For example, the light-emitting layer comprising a thin stratum of organic pigment molecules and possibly conductive organic polymers may be embedded between two further electrically conductive organic layers which transport charge carriers from the two electrodes to the light-emitting layer. The electrically conductive organic layer between the light-emitting layer and the cathode conducts electrons whereas the corresponding layer between the light-emitting layer and the anode conducts holes.
The use of such organic charge carrier transport layers, however, also involves problems. The thermal load on the layers during operation and material interactions between the electrode layers and the transport layers lead to a deterioration of the luminous efficacy of the component in the course of time. The useful life of the organic electron-conducting layer is very short in this case, in particular when strongly reducing metals such as calcium or magnesium, which have a particularly low work function for the electrons, are used as the cathode material in order to achieve a high luminous efficacy.
It is accordingly proposed in U.S. Pat. No. 5,128,587 to choose a composition of an organic or alternatively inorganic semiconductor for the charge transport layer which transports electrons and lies between the electrode with low work function and the luminescent film. Inorganic semiconductors proposed here are Ge, Si, Sn, SiC, AlSb, BN, BP, GaN, GaSb, GaAs, GaP, InSb, InAs, InP, CdSe, CdTe, ZnO, ZnS, or ZnSe. The semiconducting layer may be amorphous or crystalline and it may be an N-type doped semiconductor or an intrinsic semiconductor.
A disadvantage of a component having a charge transport layer with an inorganic semiconductor of the kind mentioned above is that this layer absorbs light in the visible spectrum range.